The present invention relates to an image reading apparatus for reading the image of transmission-type originals such as negative films and reversal films. More particularly, the present invention relates to an image reading apparatus that is capable of reading images on such transmission-type originals with high efficiency and rapidity while assuring high precision of image reading.
One of the recent advances in image forming technology is the development of digital photoprinters in which image information recorded on photographic films such as negative films and reversal films is read photoelectrically and the image thus read is converted to a digital signal, which is then subjected to various image processing operations to produce recording image information and in which a light-sensitive material such as photographic paper is scan exposed with recording light that has been modulated in accordance with said image information, thereby producing finished prints.
With digital photoprinters, the layout of editing operations on printed images such as composing more than one image, dividing an image and editing both characters and images, as well as various image processing operations such as color/density adjustment, scaling and edge enhancement can be performed freely so as to produce finished prints that have been freely edited and an image processed in accordance with the intended use.
It should also be noted that compared to conventional areally exposed prints, not all of the image density information that is recorded on films can be reproduced for various reasons associated with density resolution, spatial resolution, color/density reproduction, etc. However, digital photoprinters are capable of outputting prints that assure almost 100% reproduction of the image density information that is recorded on the films.
A digital photoprinter which has such advantageous features is composed of the following basic parts: a reading apparatus that reads the image as recorded on a film, a setup apparatus in which the image thus read is subjected to processing so as to determine the conditions for subsequent exposure, and an image forming apparatus that scan exposes a light-sensitive material in accordance with the thus determined conditions for exposure and which then performs development and subsequent processing of the exposed light-sensitive material.
In the apparatus for reading the image recorded on a film, typically in the case of image reading by slit scanning, the film is illuminated with slit reading light which extends in a one-dimensional direction while, at the same time, the film is moved in a direction generally perpendicular to the one-dimensional direction (alternatively, both the reading light and a photoelectric transducer device are moved), whereby the film is scanned in two-dimensions with the reading light.
The light that has passed through the film and which carries the image on it is focused on the light-receiving surface of a photoelectric transducer device such as a charge-coupled device (hereunder abbreviated as "CCD") line sensor, where the light is subjected to photoelectric conversion for image reading.
The image information thus read is amplified, converted to a digital signal and subjected to various image processing operations such as correction of characteristic errors in the CCD device, changing the density and scaling before transfer to the setup apparatus.
In the setup apparatus, the transferred image information is reproduced as a visible image on a display such as a CRT (cathode-ray tube). The operator who is looking at the reproduced image may perform any necessary additional corrections such as gray-level or gradation correction and color/density correction. When the reproduced image is acceptable (passes the verification), it is transferred as recording image information to the image forming apparatus.
If the image forming apparatus is of a type that uses raster scanning (scanning by light beam) for recording image, three light beams that are capable of exposure for three primary colors, (R, G and B) are modulated in accordance with the above-mentioned recording image information so that they are deflected in a fast scan direction (corresponding to the above-mentioned one-dimensional direction) while, at the same time, the light-sensitive material is transported for slow scanning in a direction generally perpendicular to the fast scan direction (i.e., the deflected light beams and the light-sensitive material are slow scanned relative to each other), whereby the light-sensitive material is scan exposed in two-dimensions with light beams that have been modulated in accordance with the recorded image, with the thus read image on the film being recorded on the light-sensitive material.
The exposed light-sensitive material is then subjected to development and subsequent processing in accordance with the type of light-sensitive material. In case of silver halide photography, the light-sensitive material is subjected to processing that includes color development, bleach fixing, washing, drying, etc., and the finished print emerges from the image forming apparatus.
As is well known, films are not always exposed to an appropriate amount of light and the exposure may be insufficient (underexposed) or excessive (overexposed).
Further, the image recorded on negative films usually has a density D (log E) range of about 3.2, whereas the image recorded on reversal films has a wider density range of about 3.8.
In order to produce finished prints of high quality with digital photoprinters, it is necessary to use a photoelectric transducer that has high resolving power in terms of both space and density (light quantity), and for this purpose a CCD sensor is typically used with advantage. However, a photoelectric transducer having high spatial and density resolving powers is generally limited in dynamic range (the range of densities that can be measured), and it is difficult to measure densities over the whole range of negative films (D.congruent.3.2) or reversal films (D.congruent.3.8).
In order to realize precise image reading by effective operation of the photoelectric transducer employed in the reading apparatus of a digital photoprinter, it is necessary to provide the normal amount of exposure (the product of light quantity and measurement time) that is appropriate for the particular photoelectric transducer. Therefore, if the quantity of reading light and the reading time are set in accordance with an underexposed image which transmits a large amount of light, the amount of light received by the photoelectric transducer means is insufficient for other images to be read correctly. To avoid this problem, the quantity of reading light and the reading time are set in accordance with films that will transmit theoretically the least possible amount of light (i.e., overexposed films-having the highest density) and precise reading of the image on other films is insured by adjusting the quantities of the reading light and the transmitted light with the aid of optical filters or imaging lens opening.
However, this practice requires not only optical members such as optical filters and an imaging lens, but also an apparatus for driving and adjusting such optical members. As a result, the reading unit becomes complex and bulky, leading to a higher cost of the digital photoprinter. Further, the measurement time is also set in accordance with overexposed films which require the longest measurement, and hence the efficiency of reading the image on films is too low to accomplish rapid emergence of finished prints.
In addition, as already mentioned, a photoelectric transducer having high spatial and density resolving powers is limited in dynamic range and it is incapable of reading information over the whole density range of the image recorded on negative or reversal films. However, in order to achieve satisfactory reproduction of the image that is carried on a film, one needs only to read the image information at a density in the range of about 2.0 depending upon the state of exposure of the film (i.e., whether it is correctly exposed, underexposed or overexposed).
Under these circumstances, in order to determine the range of densities that have to be read by the photoelectric transducer in an image reading apparatus of a type typically used in a digital photoprinter, the reading of the image on the film for printing is preceded by prescanning, which involves rough reading of the image on the film with the dynamic range of the photoelectric transducer being rendered effectively wide, and the range of densities to be read by the CCD sensor during main scanning is determined in accordance with the result of the prescanning operation.
Thus, a digital photoprinter reads the image on the film by a procedure that consists, in sequence, of prescanning, determination of the range of densities that need be read, and main scanning. With the conventional system, prescanning is followed by an intermediate step in which the film or the reading light source and the CCD sensor are returned to the state where they are ready for the start of another reading cycle before main scanning is effected. This further reduces the efficiency of reading the image on the film and, in addition, the movement of the film (or the combination of the reading light and the CCD sensor) becomes so complicated that not only the structural complexity but also the cost of the image forming apparatus is increased.